Miniaturization and functional diversification of electronic equipment are increasingly requiring high density printed wiring boards to be used for the electronic equipment.
Multilayering is a common technique for increasing the density of printed wiring boards. A multilayer printed wiring board is most commonly prepared by laminating an electrical insulation layer on the surface of an internal layer substrate of which the outermost layer is a conductive layer (a first conductive layer) and forming a new conductive layer (a second conductive layer) on the electrical insulation layer. Optionally, several electrical insulation layers and conductive layers are additionally laminated.
In this type of multilayer printed wiring board, adhesion (pattern adhesion) of the electrical insulation layer with the conductive pattern of the second conductive layer formed thereon is important to ensure a long life. A method of roughening the electrical insulation layer disclosed in Japanese Patent No. 2877110, for example, is widely employed for obtaining the pattern adhesion. Coating the roughened electrical insulation layer with an adhesive for electroless plating containing polymer components such as a rubber and resin to improve the pattern adhesion has been studied (Japanese patent Application Laid-open 2001-192844, etc.).
However, the roughening or coating with an adhesive after forming the electrical insulation layer cannot necessarily ensure sufficient pattern adhesion when there is a change in the temperature and humidity. There have been cases in which the life of multilayer printed wiring board is reduced.